Port old entity spawn code

[codelucas]

After Allan's commit in 9f6c01caaea020771567adcd1ac9a811c9d2690b line 326, I noticed that he used a Phaser feature I was not aware of, the ability to call methods repeatedly on the game clock every X seconds.

We should probably port out existing entity spawning code (which uses modulus math on the game seconds) to this to give us greater control of our entity spawn rates.

[codelucas]

Never-mind, this issue is on hold for now because it may conflict with: https://github.com/CS113/fatty-big-star/issues/6

Which happens to be a more important issue!

[codelucas]

Never mind again, as per brought up by @skylarbpayne, this is actually a really big issue.

[codelucas]

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